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Network Working Group                                           C. Rigney
Request for Comments: 2869                                     Livingston
Category: Informational                                        W. Willats
                                                        Cyno Technologies
                                                               <a href='#sP.'>P.</a> Calhoun
                                                         Sun Microsystems
                                                                June 2000


                           RADIUS Extensions

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes additional attributes for carrying
   authentication, authorization and accounting information between a
   Network Access Server (NAS) and a shared Accounting Server using the
   Remote Authentication Dial In User Service (RADIUS) protocol
   described in <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] and <A HREF="http://RFC.net/rfc2866.html">RFC 2866</A> [2].

Table of Contents

   <a href='#s1.'>1.</a>     Introduction ..........................................    <a href='#p2'>2</a>
      <a href='#s1.1'>1.1</a>       Specification of Requirements ...................    <a href='#p3'>3</a>
      <a href='#s1.2'>1.2</a>       Terminology .....................................    <a href='#p3'>3</a>
   <a href='#s2.'>2.</a>     Operation .............................................    <a href='#p4'>4</a>
      <a href='#s2.1'>2.1</a>       RADIUS support for Interim Accounting Updates....    4
      <a href='#s2.2'>2.2</a>       RADIUS support for Apple Remote Access
                Protocol ........................................    <a href='#p5'>5</a>
      <a href='#s2.3'>2.3</a>       RADIUS Support for Extensible Authentication
                Protocol (EAP) ..................................   <a href='#p11'>11</a>
         <a href='#s2.3.1'>2.3.1</a>  Protocol Overview ...............................   <a href='#p11'>11</a>
         <a href='#s2.3.2'>2.3.2</a>  Retransmission ..................................   <a href='#p13'>13</a>
         <a href='#s2.3.3'>2.3.3</a>  Fragmentation ...................................   <a href='#p14'>14</a>
         <a href='#s2.3.4'>2.3.4</a>  Examples ........................................   <a href='#p14'>14</a>
         <a href='#s2.3.5'>2.3.5</a>  Alternative uses ................................   <a href='#p19'>19</a>
   <a href='#s3.'>3.</a>     Packet Format .........................................   <a href='#p19'>19</a>
   <a href='#s4.'>4.</a>     Packet Types ..........................................   <a href='#p19'>19</a>
   <a href='#s5.'>5.</a>     Attributes ............................................   <a href='#p20'>20</a>



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      <a href='#s5.1'>5.1</a>       Acct-Input-Gigawords ............................   <a href='#p22'>22</a>
      <a href='#s5.2'>5.2</a>       Acct-Output-Gigawords ...........................   <a href='#p23'>23</a>
      <a href='#s5.3'>5.3</a>       Event-Timestamp .................................   <a href='#p23'>23</a>
      <a href='#s5.4'>5.4</a>       ARAP-Password ...................................   <a href='#p24'>24</a>
      <a href='#s5.5'>5.5</a>       ARAP-Features ...................................   <a href='#p25'>25</a>
      <a href='#s5.6'>5.6</a>       ARAP-Zone-Access ................................   <a href='#p26'>26</a>
      <a href='#s5.7'>5.7</a>       ARAP-Security ...................................   <a href='#p27'>27</a>
      <a href='#s5.8'>5.8</a>       ARAP-Security-Data ..............................   <a href='#p28'>28</a>
      <a href='#s5.9'>5.9</a>       Password-Retry ..................................   <a href='#p28'>28</a>
      <a href='#s5.10'>5.10</a>      Prompt ..........................................   <a href='#p29'>29</a>
      <a href='#s5.11'>5.11</a>      Connect-Info ....................................   <a href='#p30'>30</a>
      <a href='#s5.12'>5.12</a>      Configuration-Token .............................   <a href='#p31'>31</a>
      <a href='#s5.13'>5.13</a>      EAP-Message .....................................   <a href='#p32'>32</a>
      <a href='#s5.14'>5.14</a>      Message-Authenticator ...........................   <a href='#p33'>33</a>
      <a href='#s5.15'>5.15</a>      ARAP-Challenge-Response .........................   <a href='#p35'>35</a>
      <a href='#s5.16'>5.16</a>      Acct-Interim-Interval ...........................   <a href='#p36'>36</a>
      <a href='#s5.17'>5.17</a>      NAS-Port-Id .....................................   <a href='#p37'>37</a>
      <a href='#s5.18'>5.18</a>      Framed-Pool .....................................   <a href='#p37'>37</a>
      <a href='#s5.19'>5.19</a>      Table of Attributes .............................   <a href='#p38'>38</a>
   <a href='#s6.'>6.</a>     IANA Considerations ...................................   <a href='#p39'>39</a>
   <a href='#s7.'>7.</a>     Security Considerations ...............................   <a href='#p39'>39</a>
      <a href='#s7.1'>7.1</a>       Message-Authenticator Security ..................   <a href='#p39'>39</a>
      <a href='#s7.2'>7.2</a>       EAP Security ....................................   <a href='#p39'>39</a>
         <a href='#s7.2.1'>7.2.1</a>  Separation of EAP server and PPP authenticator ..   40
         <a href='#s7.2.2'>7.2.2</a>  Connection hijacking ............................   <a href='#p41'>41</a>
         <a href='#s7.2.3'>7.2.3</a>  Man in the middle attacks .......................   <a href='#p41'>41</a>
         <a href='#s7.2.4'>7.2.4</a>  Multiple databases ..............................   <a href='#p41'>41</a>
         <a href='#s7.2.5'>7.2.5</a>  Negotiation attacks .............................   <a href='#p42'>42</a>
   <a href='#s8.'>8.</a>     References ............................................   <a href='#p43'>43</a>
   <a href='#s9.'>9.</a>     Acknowledgements ......................................   <a href='#p44'>44</a>
   <a href='#s10.'>10.</a>    Chair's Address .......................................   <a href='#p44'>44</a>
   <a href='#s11.'>11.</a>    Authors' Addresses ....................................   <a href='#p45'>45</a>
   <a href='#s12.'>12.</a>    Full Copyright Statement ..............................   <a href='#p47'>47</a>

<h2><a name='s1.'>1.  Introduction</a></h2>

   <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] describes the RADIUS Protocol as it is implemented and
   deployed today, and <A HREF="http://RFC.net/rfc2866.html">RFC 2866</A> [2] describes how Accounting can be
   performed with RADIUS.












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   This memo suggests several additional Attributes that can be added to
   RADIUS to perform various useful functions.  These Attributes do not
   have extensive field experience yet and should therefore be
   considered experimental.

   The Extensible Authentication Protocol (EAP) [3] is a PPP extension
   that provides support for additional authentication methods within
   <a href='#sPPP.'>PPP.</a>  This memo describes how the EAP-Message and Message-
   Authenticator attributes may be used for providing EAP support within
   RADIUS.

   All attributes are comprised of variable length Type-Length-Value 3-
   tuples.  New attribute values can be added without disturbing
   existing implementations of the protocol.

<h2><a name='s1.1.'>1.1.  Specification of Requirements</a></h2>

   The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL NOT&quot;,
   &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,  &quot;MAY&quot;, and &quot;OPTIONAL&quot; in this
   document are to be interpreted as described in <A HREF="http://RFC.net/rfc2119.html">RFC 2119</A> [4].

   An implementation is not compliant if it fails to satisfy one or more
   of the must or must not requirements for the protocols it implements.
   An implementation that satisfies all the must, must not, should and
   should not requirements for its protocols is said to be
   &quot;unconditionally compliant&quot;; one that satisfies all the must and must
   not requirements but not all the should or should not requirements
   for its protocols is said to be &quot;conditionally compliant.&quot;

   A NAS that does not implement a given service MUST NOT implement the
   RADIUS attributes for that service.  For example, a NAS that is
   unable to offer ARAP service MUST NOT implement the RADIUS attributes
   for ARAP.  A NAS MUST treat a RADIUS access-request requesting an
   unavailable service as an access-reject instead.

<h2><a name='s1.2.'>1.2.  Terminology</a></h2>

   This document uses the following terms:

   service   The NAS provides a service to the dial-in user, such as PPP
             or Telnet.

   session   Each service provided by the NAS to a dial-in user
             constitutes a session, with the beginning of the session
             defined as the point where service is first provided and
             the end of the session defined as the point where service





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             is ended.  A user may have multiple sessions in parallel or
             series if the NAS supports that, with each session
             generating a separate start and stop accounting record.

   silently discard
             This means the implementation discards the packet without
             further processing.  The implementation SHOULD provide the
             capability of logging the error, including the contents of
             the silently discarded packet, and SHOULD record the event
             in a statistics counter.

<h2><a name='s2.'>2.  Operation</a></h2>

   Operation is identical to that defined in <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] and <A HREF="http://RFC.net/rfc2866.html">RFC 2866</A>
   [2].

<h2><a name='s2.1.'>2.1.  RADIUS support for Interim Accounting Updates</a></h2>

   When a user is authenticated, a RADIUS server issues an Access-Accept
   in response to a successful Access-Request. If the server wishes to
   receive interim accounting messages for the given user it must
   include the Acct-Interim-Interval RADIUS attribute in the message,
   which indicates the interval in seconds between interim messages.

   It is also possible to statically configure an interim value on the
   NAS itself. Note that a locally configured value on the NAS MUST
   override the value found in an Access-Accept.

   This scheme does not break backward interoperability since a RADIUS
   server not supporting this extension will simply not add the new
   Attribute. NASes not supporting this extension will ignore the
   Attribute.

   Note that all information in an interim message is cumulative (i.e.
   number of packets sent is the total since the beginning of the
   session, not since the last interim message).

   It is envisioned that an Interim Accounting record (with Acct-
   Status-Type = Interim-Update (3)) would contain all of the attributes
   normally found in an Accounting Stop message with the exception of
   the Acct-Term-Cause attribute.

   Since all the information is cumulative, a NAS MUST ensure that only
   a single generation of an interim Accounting message for a given
   session is present in the retransmission queue at any given time.






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   A NAS MAY use a fudge factor to add a random delay between Interim
   Accounting messages for separate sessions. This will ensure that a
   cycle where all messages are sent at once is prevented, such as might
   otherwise occur if a primary link was recently restored and many
   dial-up users were directed to the same NAS at once.

   The Network and NAS CPU load of using Interim Updates should be
   carefully considered, and appropriate values of Acct-Interim-Interval
   chosen.

<h2><a name='s2.2.'>2.2.  RADIUS support for Apple Remote Access Protocol</a></h2>

   The RADIUS (Remote Authentication Dial-In User Service) protocol
   provides a method that allows multiple dial-in Network Access Server
   (NAS) devices to share a common authentication database.

   The Apple Remote Access Protocol (ARAP) provides a method for sending
   AppleTalk network traffic over point-to-point links, typically, but
   not exclusively, asynchronous and ISDN switched-circuit connections.
   Though Apple is moving toward ATCP on PPP for future remote access
   services, ARAP is still a common way for the installed base of
   Macintosh users to make remote network connections, and is likely to
   remain so for some time.

   ARAP is supported by several NAS vendors who also support PPP, IPX
   and other protocols in the same NAS. ARAP connections in these
   multi-protocol devices are often not authenticated with RADIUS, or if
   they are, each vendor creates an individual solution to the problem.

   This section describes the use of additional RADIUS attributes to
   support ARAP. RADIUS client and server implementations that implement
   this specification should be able to authenticate ARAP connections in
   an interoperable manner.

   This section assumes prior knowledge of RADIUS, and will go into some
   detail on the operation of ARAP before entering a detailed discussion
   of the proposed ARAP RADIUS attributes.

   There are two features of ARAP this document does not address:

      <a href='#s1.'>1.</a> User initiated password changing. This is not part of RADIUS,
         but can be implemented through a software process other than
         RADIUS.

      <a href='#s2.'>2.</a> Out-of-Band messages. At any time, the NAS can send messages to
         an ARA client which appear in a dialog box on the dial-in
         user's screen.  These are not part of authentication and do not
         belong here. However, we note that a Reply-Message attribute in



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         an Access-Accept may be sent down to the user as a sign-on
         message of the day string using the out-of-band channel.

   We have tried to respect the spirit of the existing RADIUS protocol
   as much as possible, making design decisions compatible with prior
   art.  Further, we have tried to strike a balance between flooding the
   RADIUS world with new attributes, and hiding all of ARAP operation
   within a single multiplexed ARAP attribute string or within Extended
   Authentication Protocol (EAP) [3] machinery.

   However, we feel ARAP is enough of a departure from PPP to warrant a
   small set of similarly named attributes of its own.

   We have assumed that an ARAP-aware RADIUS server will be able to do
   DES encryption and generate security module challenges.  This is in
   keeping with the general RADIUS goal of smart server / simple NAS.

   ARAP authenticates a connection in two phases. The first is a &quot;Two-
   Way DES&quot; random number exchange, using the user's password as a key.
   We say &quot;Two-Way&quot; because the ARAP NAS challenges the dial-in client
   to authenticate itself, and the dial-in client challenges the ARAP
   NAS to authenticate itself.

   Specifically, ARAP does the following:

      <a href='#s1.'>1.</a> The NAS sends two 32-bit random numbers to the dial-in client
         in an ARAP msg_auth_challenge packet.

      <a href='#s2.'>2.</a> The dial-in client uses the user's password to DES encrypt the
         two random numbers sent to it by the NAS. The dial-in client
         then sends this result, the user's name and two 32-bit random
         numbers of its own back to the NAS in an ARAP msg_auth_request
         packet.

      <a href='#s3.'>3.</a> The NAS verifies the encrypted random numbers sent by the
         dial-in client are what it expected. If so, it encrypts the
         dial-in client's challenge using the password and sends it back
         to the dial-in client in an ARAP msg_auth_response packet.

   Note that if the dial-in client's response was wrong,  meaning the
   user has the wrong password, the server can initiate a retry sequence
   up to the maximum amount of retries allowed by the NAS. In this case,
   when the dial-in client receives the ARAP msg_auth_response packet it
   will acknowledge it with an ARAP msg_auth_again packet.

   After this first &quot;DES Phase&quot; the ARAP NAS MAY initiate a secondary
   authentication phase using what Apple calls &quot;Add-In Security
   Modules.&quot;  Security Modules are small pieces of code which run on



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   both the client and server and are allowed to read and write
   arbitrary data across the communications link to perform additional
   authentication functions.  Various security token vendors use this
   mechanism to authenticate ARA callers.

   Although ARAP allows security modules to read and write anything they
   like, all existing security modules use simple challenge and response
   cycles, with perhaps some overall control information.  This document
   assumes all existing security modules can be supported with one or
   more challenge/response cycles.

   To complicate RADIUS and ARAP integration, ARAP sends down some
   profile information after the DES Phase and before the Security
   Module phase.  This means that besides the responses to challenges,
   this profile information must also be present, at somewhat unusual
   times.  Fortunately the information is only a few  pieces of numeric
   data related to passwords, which this document packs into a single
   new attribute.

   Presenting an Access-Request to RADIUS on behalf of an ARAP
   connection is straightforward. The ARAP NAS generates the random
   number challenge, and then receives the dial-in client's response,
   the dial-in client's challenge, and the user's name. Assuming the
   user is not a guest, the following information is forwarded in an
   Access-Request packet:  User-Name (up to 31 characters long),
   Framed-Protocol (set to 3, ARAP), ARAP-Password, and any additional
   attributes desired, such as Service-Type, NAS-IP-Address, NAS-Id,
   NAS-Port-Type, NAS-Port, NAS-Port-Id, Connect-Info, etc.

   The Request Authenticator is a NAS-generated 16 octet random number.
   The low-order 8 octets of this number are sent to the dial-in user as
   the two 4 octet random numbers required in the ARAP
   msg_auth_challenge packet. Octets 0-3 are the first random number and
   Octets 4-7 are the second random number.

   The ARAP-Password in the Access-Request contains a 16 octet random
   number field, and is used to carry the dial-in user's response to the
   NAS challenge and the client's own challenge to the NAS.  The high-
   order octets contain the dial-in user's challenge to the NAS (2 32-
   bit numbers, 8 octets) and the low-order octets contain the dial-in
   user's response to the NAS challenge (2 32-bit numbers, 8 octets).

   Only one of User-Password, CHAP-Password, or ARAP-Password needs to
   be present in an Access-Request, or one or more EAP-Messages.

   If the RADIUS server does not support ARAP it SHOULD return an
   Access-Reject to the NAS.




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   If the RADIUS server does support ARAP, it should verify the user's
   response using the Challenge (from the lower order 8 octets of the
   Request Authenticator) and the user's response (from the low order 8
   octets of the ARAP-Password).

   If that authentication fails, the RADIUS server should return an
   Access-Reject packet to the NAS, with optional Password-Retry and
   Reply-Messages attributes.  The presence of Password-Retry indicates
   the ARAP NAS MAY choose to initiate another challenge-response cycle,
   up to a total number of times equal to the integer value of the
   Password-Retry attribute.

   If the user is authenticated, the RADIUS server should return an
   Access-Accept packet (Code 2) to the NAS, with ID and Response
   Authenticator as usual, and attributes as follows:

      Service-Type of Framed-Protocol.

      Framed-Protocol of ARAP (3).

      Session-Timeout with the maximum connect time for the user in
      seconds.  If the user is to be given unlimited time,
      Session-Timeout should not be included in the Access-Accept
      packet, and ARAP will treat that as an unlimited timeout (-1).

      ARAP-Challenge-Response, containing 8 octets with the response to
      the dial-in client's challenge. The RADIUS server calculates this
      value by taking the dial-in client's challenge from the high order
      8 octets of the ARAP-Password attribute and  performing DES
      encryption on this value with the authenticating user's password
      as the key. If the user's password is less than 8 octets in
      length, the password is padded at the end with NULL octets to a
      length of 8 before using it as a key. If the user's password is
      greater than 8 octets in length, an Access-Reject MUST be sent
      instead.

      ARAP-Features, containing information that the NAS should send to
      the user in an ARAP &quot;feature flags&quot; packet.

         Octet 0: If zero, user cannot change their password. If non-
         zero user can.  (RADIUS does not handle the password changing,
         just the attribute which indicates whether ARAP indicates they
         can.)

         Octet 1: Minimum acceptable password length (0-8).






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         Octet 2-5: Password creation date in Macintosh format, defined
         as 32 bits unsigned representing seconds since Midnight GMT
         January 1, 1904.

         Octet 6-9 Password Expiration Delta from create date in
         seconds.

         Octet 10-13: Current RADIUS time in Macintosh format

      Optionally, a single Reply-Message with a text string up to 253
      characters long which MAY be sent down to the user to be displayed
      in a sign-on/message of the day dialog.

      Framed-AppleTalk-Network may be included.

      Framed-AppleTalk-Zone, up to 32 characters in length, may be
      included.

      ARAP defines the notion of a list of zones for a user.  Along with
      a list of zone names, a Zone Access Flag is defined (and used by
      the NAS) which says how to use the list of zone names. That is,
      the dial-in user may only be allowed to see the Default Zone, or
      only the zones in the zone list (inclusive) or any zone except
      those in the zone list (exclusive).

      The ARAP NAS handles this by having a named filter which contains
      (at least) zone names.  This solves the problem where a single
      RADIUS server is managing disparate NAS clients who may not be
      able to &quot;see&quot; all of the zone names in a user zone list.  Zone
      names only have meaning &quot;at the NAS.&quot; The disadvantage of this
      approach is that zone filters must be set up on the NAS somehow,
      then referenced by the RADIUS Filter-Id.

      ARAP-Zone-Access contains an integer which specifies how the &quot;zone
      list&quot; for this user should be used.  If this attribute is present
      and the value is 2 or 4 then a Filter-Id must also be present to
      name a zone list filter to apply the access flag to.

      The inclusion of a Callback-Number or Callback-Id attribute in the
      Access-Accept MAY cause the ARAP NAS to disconnect after sending
      the Feature Flags to begin callback processing in an ARAP specific
      way.









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   Other attributes may be present in the Access-Accept packet as well.

   An ARAP NAS will need other information to finish bringing up the
   connection to the dial in client, but this information can be
   provided by the ARAP NAS without any help from RADIUS, either through
   configuration by SNMP, a NAS administration program, or deduced by
   the AppleTalk stack in the NAS. Specifically:

      <a href='#s1.'>1.</a> AppearAsNet and AppearAsNode values, sent to the client to tell
         it what network and node numbers it should use in its datagram
         packets.  AppearAsNet can be taken from the Framed-AppleTalk-
         Network attribute or from the configuration or AppleTalk stack
         onthe NAS.

      <a href='#s2.'>2.</a> The &quot;default&quot; zone - that is the name of the AppleTalk zone in
         which the dial-in client will appear.  (Or can be specified
         with the Framed-AppleTalk-Zone attribute.)

      <a href='#s3.'>3.</a> Other very NAS specific stuff such as the name of the NAS, and
         smartbuffering information.  (Smartbuffering is an ARAP
         mechanism for replacing common AppleTalk datagrams with small
         tokens, to improve slow link performance in a few common
         traffic situations.)

      <a href='#s4.'>4.</a> &quot;Zone List&quot; information for this user.  The ARAP specification
         defines a &quot;zone count&quot; field which is actually unused.

   RADIUS supports ARAP Security Modules in the following manner.

   After DES authentication has been completed, the RADIUS server may
   instruct the ARAP NAS to run one or more security modules for the
   dial-in user. Although the underlying protocol supports executing
   multiple security modules in series, in practice all current
   implementations only allow executing one.  Through the use of
   multiple Access-Challenge requests, multiple modules can be
   supported, but this facility will probably never be used.

   We also assume that, even though ARAP allows a free-form dialog
   between security modules on each end of the point-to-point link, in
   actual practice all security modules can be reduced to a simple
   challenge/response cycle.

   If the RADIUS server wishes to instruct the ARAP NAS to run a
   security module, it should send an Access-Challenge packet to the NAS
   with (optionally) the State attribute, plus the ARAP-Challenge-
   Response, ARAP-Features, and two more attributes:





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   ARAP-Security: a four octet security module signature, containing a
   Macintosh OSType.

   ARAP-Security-Data, a string to carry the actual security module
   challenge and response.

   When the security module finishes executing, the security module
   response is passed  in an ARAP-Security-Data attribute from the NAS
   to the RADIUS server in a second Access-Request, also including the
   State from the Access-Challenge.  The authenticator field contains no
   special information in this case, and this can be discerned by the
   presence of the State attribute.

<h2><a name='s2.3.'>2.3.  RADIUS Support for Extensible Authentication Protocol (EAP)</a></h2>

   The Extensible Authentication Protocol (EAP), described in [3],
   provides a standard mechanism for support of additional
   authentication methods within PPP.  Through the use of EAP, support
   for a number of authentication schemes may be added, including smart
   cards, Kerberos, Public Key, One Time Passwords, and others.  In
   order to provide for support of EAP within RADIUS, two new
   attributes, EAP-Message and Message-Authenticator, are introduced in
   this document. This section describes how these new attributes may be
   used for providing EAP support within RADIUS.

   In the proposed scheme, the RADIUS server is used to shuttle RADIUS-
   encapsulated EAP Packets between the NAS and a backend security
   server. While the conversation between the RADIUS server and the
   backend security server will typically occur using a proprietary
   protocol developed by the backend security server vendor, it is also
   possible to use RADIUS-encapsulated EAP via the EAP-Message
   attribute.  This has the advantage of allowing the RADIUS server to
   support EAP without the need for authentication-specific code, which
   can instead reside on the backend security server.

<h2><a name='s2.3.1.'>2.3.1.  Protocol Overview</a></h2>

   The EAP conversation between the authenticating peer (dial-in user)
   and the NAS begins with the negotiation of EAP within LCP.  Once EAP
   has been negotiated, the NAS MUST send an EAP-Request/Identity
   message to the authenticating peer, unless identity is determined via
   some other means such as Called-Station-Id or Calling-Station-Id.
   The peer will then respond with an EAP-Response/Identity which the
   the NAS will then forward to the RADIUS server in the EAP-Message
   attribute of a RADIUS Access-Request packet. The RADIUS Server will
   typically use the EAP-Response/Identity to determine which EAP type
   is to be applied to the user.




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   In order to permit non-EAP aware RADIUS proxies to forward the
   Access-Request packet, if the NAS sends the EAP-Request/Identity, the
   NAS MUST copy the contents of the EAP-Response/Identity into the
   User-Name attribute and MUST include the EAP-Response/Identity in the
   User-Name attribute in every subsequent Access-Request. NAS-Port or
   NAS-Port-Id SHOULD be included in the attributes issued by the NAS in
   the Access-Request packet, and either NAS-Identifier or NAS-IP-
   Address MUST be included.  In order to permit forwarding of the
   Access-Reply by EAP-unaware proxies, if a User-Name attribute was
   included in an Access-Request, the RADIUS Server MUST include the
   User-Name attribute in subsequent Access-Accept packets. Without the
   User-Name attribute, accounting and billing becomes very difficult to
   manage.

   If identity is determined via another means such as Called-Station-Id
   or Calling-Station-Id, the NAS MUST include these identifying
   attributes in every Access-Request.

   While this approach will save a round-trip, it cannot be universally
   employed.  There are circumstances in which the user's identity may
   not be needed (such as when authentication and accounting is handled
   based on Called-Station-Id or Calling-Station-Id), and therefore an
   EAP-Request/Identity packet may not necessarily be issued by the NAS
   to the authenticating peer. In cases where an EAP-Request/Identity
   packet will not be sent, the NAS will send to the RADIUS server a
   RADIUS Access-Request packet containing an EAP-Message attribute
   signifying EAP-Start. EAP-Start is indicated by sending an EAP-
   Message attribute with a length of 2 (no data). However, it should be
   noted that since no User-Name attribute is included in the Access-
   Request, this approach is not compatible with RADIUS as specified in
   [1], nor can it easily be applied in situations where proxies are
   deployed, such as roaming or shared use networks.

   If the RADIUS server supports EAP, it MUST respond with an Access-
   Challenge packet containing an EAP-Message attribute. If the RADIUS
   server does not support EAP, it MUST respond with an Access-Reject.
   The EAP-Message attribute includes an encapsulated EAP packet which
   is then passed on to the authenticating peer.  In the case where the
   NAS does not initially send an EAP-Request/Identity message to the
   peer, the Access-Challenge typically will contain an EAP-Message
   attribute encapsulating an EAP-Request/Identity message, requesting
   the dial-in user to identify themself. The NAS will then respond with
   a RADIUS Access-Request packet containing an EAP-Message attribute
   encapsulating an EAP-Response.  The conversation continues until
   either a RADIUS Access-Reject or Access-Accept packet is received.






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   Reception of a RADIUS Access-Reject packet, with or without an EAP-
   Message attribute encapsulating EAP-Failure, MUST result in the NAS
   issuing an LCP Terminate Request to the authenticating peer.  A
   RADIUS Access-Accept packet with an EAP-Message attribute
   encapsulating EAP-Success successfully ends the authentication phase.
   The RADIUS Access-Accept/EAP-Message/EAP-Success packet MUST contain
   all of the expected attributes which are currently returned in an
   Access-Accept packet.

   The above scenario creates a situation in which the NAS never needs
   to manipulate an EAP packet.  An alternative may be used in
   situations where an EAP-Request/Identity message will always be sent
   by the NAS to the authenticating peer.

   For proxied RADIUS requests there are two methods of processing.  If
   the domain is determined based on the Called-Station-Id, the RADIUS
   Server may proxy the initial RADIUS Access-Request/EAP-Start. If the
   domain is determined based on the user's identity, the local RADIUS
   Server MUST respond with a RADIUS Access-Challenge/EAP-Identity
   packet.  The response from the authenticating peer MUST be proxied to
   the final authentication server.

   For proxied RADIUS requests, the NAS may receive an Access-Reject
   packet in response to its Access-Request/EAP-Identity packet.  This
   would occur if the message was proxied to a RADIUS Server which does
   not support the EAP-Message extension. On receiving an Access-Reject,
   the NAS MUST send an LCP Terminate Request to the authenticating
   peer, and disconnect.

<h2><a name='s2.3.2.'>2.3.2.  Retransmission</a></h2>

   As noted in [3], the EAP authenticator (NAS) is responsible for
   retransmission of packets between the authenticating peer and the
   <a href='#sNAS.'>NAS.</a>  Thus if an EAP packet is lost in transit between the
   authenticating peer and the NAS (or vice versa), the NAS will
   retransmit. As in RADIUS [1], the RADIUS client is responsible for
   retransmission of packets between the RADIUS client and the RADIUS
   server.

   Note that it may be necessary to adjust retransmission strategies and
   authentication timeouts in certain cases. For example, when a token
   card is used additional time may be required to allow the user to
   find the card and enter the token. Since the NAS will typically not
   have knowledge of the required parameters, these need to be provided
   by the RADIUS server. This can be accomplished by inclusion of
   Session-Timeout and Password-Retry attributes within the Access-
   Challenge packet.




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   If Session-Timeout is present in an Access-Challenge packet that also
   contains an EAP-Message, the value of the Session-Timeout provides
   the NAS with the maximum number of seconds the NAS should wait for an
   EAP-Response before retransmitting the EAP-Message to the dial-in
   user.

<h2><a name='s2.3.3.'>2.3.3.  Fragmentation</a></h2>

   Using the EAP-Message attribute, it is possible for the RADIUS server
   to encapsulate an EAP packet that is larger than the MTU on the link
   between the NAS and the peer. Since it is not possible for the RADIUS
   server to use MTU discovery to ascertain the link MTU, the Framed-MTU
   attribute may be included in an Access-Request packet containing an
   EAP-Message attribute so as to provide the RADIUS server with this
   information.

<h2><a name='s2.3.4.'>2.3.4.  Examples</a></h2>

   The example below shows the conversation between the authenticating
   peer, NAS, and RADIUS server, for the case of a One Time Password
   (OTP) authentication. OTP is used only for illustrative purposes;
   other authentication protocols could also have been used, although
   they might show somewhat different behavior.

Authenticating Peer     NAS                    RADIUS Server
-------------------     ---                    -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP ACK-EAP
auth -&gt;
                        &lt;- PPP EAP-Request/
                        Identity
PPP EAP-Response/
Identity (MyID) -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/
                        EAP-Response/
                        (MyID) -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request
                                                OTP/OTP Challenge
                        &lt;- PPP EAP-Request/
                        OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw -&gt;



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                        RADIUS
                        Access-Request/
                        EAP-Message/
                        EAP-Response/
                        OTP, OTPpw -&gt;
                                                 &lt;- RADIUS
                                                 Access-Accept/
                                                 EAP-Message/EAP-Success
                                                 (other attributes)
                        &lt;- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts

In the case where the NAS first sends an EAP-Start packet to the
RADIUS server,  the conversation would appear as follows:

Authenticating Peer     NAS                    RADIUS Server
-------------------     ---                    -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP ACK-EAP
auth -&gt;
                        RADIUS
                        Access-Request/
                       EAP-Message/Start -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/Identity
                        &lt;- PPP EA-Request/
                        Identity
PPP EAP-Response/
Identity (MyID) -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/
                        EAP-Response/
                        (MyID) -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request
                                                OTP/OTP Challenge
                        &lt;- PPP EAP-Request/
                        OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw -&gt;




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                        RADIUS
                        Access-Request/
                        EAP-Message/
                        EAP-Response/
                        OTP, OTPpw -&gt;
                                                 &lt;- RADIUS
                                                 Access-Accept/
                                                 EAP-Message/EAP-Success
                                                 (other attributes)
                        &lt;- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts

In the case where the client fails EAP authentication, the
conversation would appear as follows:

Authenticating Peer     NAS                    RADIUS Server
-------------------     ---                    -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP ACK-EAP
auth -&gt;
                        Access-Request/
                        EAP-Message/Start -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/Identity
                        &lt;- PPP EAP-Request/
                        Identity
PPP EAP-Response/
Identity (MyID) -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/
                        EAP-Response/
                        (MyID) -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request
                                                OTP/OTP Challenge
                        &lt;- PPP EAP-Request/
                        OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw -&gt;
                        RADIUS
                        Access-Request/



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                        EAP-Message/
                        EAP-Response/
                        OTP, OTPpw -&gt;
                                                 &lt;- RADIUS
                                                 Access-Reject/
                                                 EAP-Message/EAP-Failure

                        &lt;- PPP EAP-Failure
                        (client disconnected)

In the case that the RADIUS server or proxy does not support
EAP-Message, the conversation would appear as follows:

Authenticating Peer     NAS                       RADIUS Server
-------------------     ---                       -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP ACK-EAP
auth -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/Start -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                        &lt;- PPP LCP Terminate
                        (User Disconnected)

In the case where the local RADIUS Server does support EAP-Message,
but the remote RADIUS Server does not, the conversation would appear
as follows:

Authenticating Peer     NAS                       RADIUS Server
-------------------     ---                       -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP ACK-EAP
auth -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/Start -&gt;
                                                  &lt;- RADIUS
                                                  Access-Challenge/
                                                  EAP-Message/Identity
                        &lt;- PPP EAP-Request/
                        Identity




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PPP EAP-Response/
Identity
(MyID) -&gt;
                        RADIUS
                        Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                                                  (proxied from remote
                                                   RADIUS Server)
                        &lt;- PPP LCP Terminate
                        (User Disconnected)

In the case where the authenticating peer does not support EAP, but
where EAP is required for that user, the conversation would appear as
follows:

Authenticating Peer     NAS                       RADIUS Server
-------------------     ---                       -------------

                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP NAK-EAP
auth -&gt;
                        &lt;- PPP LCP Request-CHAP
                        auth
PPP LCP ACK-CHAP
auth -&gt;
                        &lt;- PPP CHAP Challenge
PPP CHAP Response -&gt;
                        RADIUS
                        Access-Request/
                        User-Name,
                        CHAP-Password -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                        &lt;-  PPP LCP Terminate
                        (User Disconnected)

In the case where the NAS does not support EAP, but where EAP is
required for that user, the conversation would appear as follows:

Authenticating Peer     NAS                       RADIUS Server
-------------------     ---                       -------------

                        &lt;- PPP LCP Request-CHAP
                        auth



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PP LCP ACK-CHAP
auth -&gt;
                        &lt;- PPP CHAP Challenge
PPP CHAP Response -&gt;
                        RADIUS
                        Access-Request/
                        User-Name,
                        CHAP-Password -&gt;

                                                 &lt;- RADIUS
                                                 Access-Reject
                        &lt;-  PPP LCP Terminate
                        (User Disconnected)

<h2><a name='s2.3.5.'>2.3.5.  Alternative uses</a></h2>

   Currently the conversation between the backend security server and
   the RADIUS server is proprietary because of lack of standardization.
   In order to increase standardization and provide interoperability
   between Radius vendors and backend security vendors, it is
   recommended that RADIUS-encapsulated EAP be used for this
   conversation.

   This has the advantage of allowing the RADIUS server to support EAP
   without the need for authentication-specific  code within the RADIUS
   server. Authentication-specific code can then reside on a backend
   security server instead.

   In the case where RADIUS-encapsulated EAP is used in a conversation
   between a RADIUS server and a backend security server, the security
   server will typically return an Access-Accept/EAP-Success message
   without inclusion of the expected attributes currently returned in an
   Access-Accept. This means that the RADIUS server MUST add these
   attributes prior to sending an Access-Accept/EAP-Success message to
   the NAS.

<h2><a name='s3.'>3.  Packet Format</a></h2>

   Packet Format is identical to that defined in <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] and 2866
   [2].

<h2><a name='s4.'>4.  Packet Types</a></h2>

   Packet types are identical to those defined in <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] and 2866
   [2].

   See &quot;Table of Attributes&quot; below to determine which types of packets
   can contain which attributes defined here.



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<h2><a name='s5.'>5.  Attributes</a></h2>

   RADIUS Attributes carry the specific authentication, authorization
   and accounting details for the request and response.

   Some attributes MAY be included more than once.  The effect of this
   is attribute specific, and is specified in each attribute
   description.  The order of attributes of the same type SHOULD be
   preserved.  The order of attributes of different types is not
   required to be preserved.

   The end of the list of attributes is indicated by the Length of the
   RADIUS packet.

   A summary of the attribute format is the same as in <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1] but
   is included here for ease of reference.  The fields are transmitted
   from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      The Type field is one octet.  Up-to-date values of the RADIUS Type
      field are specified in the most recent &quot;Assigned Numbers&quot; RFC [5].
      Values 192-223 are reserved for experimental use, values 224-240
      are reserved for implementation-specific use, and values 241-255
      are reserved and should not be used.  This specification concerns
      the following values:

           1-39   (refer to <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1], &quot;RADIUS&quot;)
          40-51   (refer to <A HREF="http://RFC.net/rfc2866.html">RFC 2866</A> [2], &quot;RADIUS Accounting&quot;)
          52      Acct-Input-Gigawords
          53      Acct-Output-Gigawords
          54      Unused
          55      Event-Timestamp
          56-59   Unused
          60-63   (refer to <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A> [1], &quot;RADIUS&quot;)
          64-67   (refer to [6])
          68      (refer to [7])
          69      (refer to [6])
          70      ARAP-Password
          71      ARAP-Features
          72      ARAP-Zone-Access



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          73      ARAP-Security
          74      ARAP-Security-Data
          75      Password-Retry
          76      Prompt
          77      Connect-Info
          78      Configuration-Token
          79      EAP-Message
          80      Message-Authenticator
          81-83   (refer to [6])
          84      ARAP-Challenge-Response
          85      Acct-Interim-Interval
          86      (refer to [7])
          87      NAS-Port-Id
          88      Framed-Pool
          89      Unused
          90-91   (refer to [6])
          92-191  Unused

   Length

      The Length field is one octet, and indicates the length of this
      attribute including the Type, Length and Value fields.  If an
      attribute is received in a packet with an invalid Length, the
      entire request should be silently discarded.

   Value

      The Value field is zero or more octets and contains information
      specific to the attribute.  The format and length of the Value
      field is determined by the Type and Length fields.

      Note that none of the types in RADIUS terminate with a NUL (hex
      00).  In particular, types &quot;text&quot; and &quot;string&quot; in RADIUS do not
      terminate with a NUL (hex 00).  The Attribute has a length field
      and does not use a terminator.  Text contains UTF-8 encoded 10646
      [8] characters and String contains 8-bit binary data.  Servers and
      servers and clients MUST be able to deal with embedded nulls.
      RADIUS implementers using C are cautioned not to use strcpy() when
      handling strings.

      The format of the value field is one of five data types.  Note
      that type &quot;text&quot; is a subset of type &quot;string.&quot;

      text      1-253 octets containing UTF-8 encoded 10646 [8]
                characters. Text of length zero (0) MUST NOT be sent;
                omit the entire attribute instead.





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      string    1-253 octets containing binary data (values 0 through
                255 decimal, inclusive). Strings of length zero (0) MUST
                NOT be sent; omit the entire attribute instead.

      address   32 bit unsigned value, most significant octet first.

      integer   32 bit unsigned value, most significant octet first.

      time      32 bit unsigned value, most significant octet first --
                   seconds since 00:00:00 UTC, January 1, 1970.

<h2><a name='s5.1.'>5.1.  Acct-Input-Gigawords</a></h2>

   Description

      This attribute indicates how many times the Acct-Input-Octets
      counter has wrapped around 2^32 over the course of this service
      being provided, and can only be present in Accounting-Request
      records where the Acct-Status-Type is set to Stop or Interim-
      Update.

   A summary of the Acct-Input-Gigawords attribute format is shown
   below.  The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      52 for Acct-Input-Gigawords.

   Length

      6

   Value

      The Value field is four octets.








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<h2><a name='s5.2.'>5.2.  Acct-Output-Gigawords</a></h2>

   Description

      This attribute indicates how many times the Acct-Output-Octets
      counter has wrapped around 2^32 in the course of delivering this
      service, and can only be present in Accounting-Request records
      where the Acct-Status-Type is set to Stop or Interim-Update.

   A summary of the Acct-Output-Gigawords attribute format is shown
   below.  The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      53 for Acct-Output-Gigawords.

   Length

      6

   Value

      The Value field is four octets.

<h2><a name='s5.3.'>5.3.  Event-Timestamp</a></h2>

   Description

      This attribute is included in an Accounting-Request packet to
      record the time that this event occurred on the NAS, in seconds
      since January 1, 1970 00:00 UTC.

   A summary of the Event-Timestamp attribute format is shown below.
   The fields are transmitted from left to right.









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      55 for Event-Timestamp

   Length

      6

   Value

      The Value field is four octets encoding an unsigned integer with
      the number of seconds since January 1, 1970 00:00 UTC.

<h2><a name='s5.4.'>5.4.  ARAP-Password</a></h2>

   Description

      This attribute is only present in an Access-Request packet
      containing a Framed-Protocol of ARAP.

      Only one of User-Password, CHAP-Password, or ARAP-Password needs
      to be present in an Access-Request, or one or more EAP-Messages.

   A summary of the ARAP-Password attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |             Value2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |             Value3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |             Value4
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type

      70 for ARAP-Password.

   Length

      18

   Value

      This attribute contains a 16 octet string, used to carry the
      dial-in user's response to the NAS challenge and the client's own
      challenge to the NAS.  The high-order octets (Value1 and Value2)
      contain the dial-in user's challenge to the NAS (2 32-bit numbers,
      8 octets) and the low-order octets (Value3 and Value4) contain the
      dial-in user's response to the NAS challenge (2 32-bit numbers, 8
      octets).

<h2><a name='s5.5.'>5.5.  ARAP-Features</a></h2>

   Description

      This attribute is sent in an Access-Accept packet with Framed-
      Protocol of ARAP, and includes password information that the NAS
      should sent to the user in an ARAP &quot;feature flags&quot; packet.

   A summary of the ARAP-Features attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Value1    |    Value2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Value3                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Value4                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Value5                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      71 for ARAP-Features.

   Length

      16



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   Value

      The Value field is a compound string containing information the
      NAS should send to the user in the ARAP &quot;feature flags&quot; packet.

         Value1: If zero, user cannot change their password. If non-zero
         user can.  (RADIUS does not handle the password changing, just
         the attribute which indicates whether ARAP indicates they can.)

         Value2: Minimum acceptable password length, from 0 to 8.

         Value3: Password creation date in Macintosh format, defined as
         32 unsigned bits representing seconds since Midnight GMT
         January 1, 1904.

         Value4: Password Expiration Delta from create date in seconds.

         Value5: Current RADIUS time in Macintosh format.

<h2><a name='s5.6.'>5.6.  ARAP-Zone-Access</a></h2>

   Description

      This attribute is included in an Access-Accept packet with
      Framed-Protocol of ARAP to indicate how the ARAP zone list for the
      user should be used.

   A summary of the ARAP-Zone-Access attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      72 for ARAP-Zone-Access.

   Length

      6





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   Value

      The Value field is four octets encoding an integer with one of the
      following values:

      1      Only allow access to default zone
      2      Use zone filter inclusively
      4      Use zone filter exclusively


      The value 3 is skipped, not because these are bit flags, but
      because 3 in some ARAP implementations means &quot;all zones&quot; which is
      the same as not specifying a list at all under RADIUS.

      If this attribute is present and the value is 2 or 4 then a
      Filter-Id must also be present to name a zone list filter to apply
      the access flag to.

<h2><a name='s5.7.'>5.7.  ARAP-Security</a></h2>

   Description

      This attribute identifies the ARAP Security Module to be used in
      an Access-Challenge packet.

   A summary of the ARAP-Security attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      73 for ARAP-Security.

   Length

      6








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   Value

      The Value field is four octets, containing an integer specifying
      the security module signature, which is a Macintosh OSType.
      (Macintosh OSTypes are 4 ascii characters cast as a 32-bit
      integer)

<h2><a name='s5.8.'>5.8.  ARAP-Security-Data</a></h2>

   Description

      This attribute contains the actual security module challenge or
      response, and can be found in Access-Challenge and Access-Request
      packets.

   A summary of the ARAP-Security-Data attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      74 for ARAP-Security-Data.

   Length

      &gt;=3

   String

      The String field contains the security module challenge or
      response associated with the ARAP Security Module specified in
      ARAP-Security.

<h2><a name='s5.9.'>5.9.  Password-Retry</a></h2>

   Description

      This attribute MAY be included in an Access-Reject to indicate how
      many authentication attempts a user may be allowed to attempt
      before being disconnected.

      It is primarily intended for use with ARAP authentication.




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   A summary of the Password-Retry attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      75 for Password-Retry.

   Length

      6

   Value

      The Value field is four octets, containing an integer specifying
      the number of password retry attempts to permit the user.

<h2><a name='s5.10.'>5.10.  Prompt</a></h2>

   Description

      This attribute is used only in Access-Challenge packets, and
      indicates to the NAS whether it should echo the user's response as
      it is entered, or not echo it.


   A summary of the Prompt attribute format is shown below.  The fields
   are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      76 for Prompt.




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   Length

      6

   Value

      The Value field is four octets.

       0      No Echo
       1      Echo

<h2><a name='s5.11.'>5.11.  Connect-Info</a></h2>

   Description

      This attribute is sent from the NAS to indicate the nature of the
      user's connection.

      The NAS MAY send this attribute in an Access-Request or
      Accounting-Request to indicate the nature of the user's
      connection.

   A summary of the Connect-Info attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Text...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      77 for Connect-Info.

   Length

      &gt;= 3

   Text

      The Text field consists of UTF-8 encoded 10646 [8] characters.
      The connection speed SHOULD be included at the beginning of the
      first Connect-Info attribute in the packet.  If the transmit and
      receive connection speeds differ, they may both be included in the
      first attribute with the transmit speed first (the speed the NAS
      modem transmits at), a slash (/), the receive speed, then
      optionally other information.



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      For example, &quot;28800 V42BIS/LAPM&quot; or &quot;52000/31200 V90&quot;

      More than one Connect-Info attribute may be present in an
      Accounting-Request packet to accommodate expected efforts by ITU
      to have modems report more connection information in a standard
      format that might exceed 252 octets.

<h2><a name='s5.12.'>5.12.  Configuration-Token</a></h2>

   Description

      This attribute is for use in large distributed authentication
      networks based on proxy.  It is sent from a RADIUS Proxy Server to
      a RADIUS Proxy Client in an Access-Accept to indicate a type of
      user profile to be used.  It should not be sent to a NAS.

   A summary of the Configuration-Token attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      78 for Configuration-Token.

   Length

      &gt;= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.










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<h2><a name='s5.13.'>5.13.  EAP-Message</a></h2>

   Description

      This attribute encapsulates Extended Access Protocol [3] packets
      so as to allow the NAS to authenticate dial-in users via EAP
      without having to understand the EAP protocol.

      The NAS places any EAP messages received from the user into one or
      more EAP attributes and forwards them to the RADIUS Server as part
      of the Access-Request, which can return EAP messages in Access-
      Challenge, Access-Accept and Access-Reject packets.

      A RADIUS Server receiving EAP messages that it does not understand
      SHOULD return an Access-Reject.

      The NAS places EAP messages received from the authenticating peer
      into one or more EAP-Message attributes and forwards them to the
      RADIUS Server within an Access-Request message.  If multiple EAP-
      Messages are contained within an Access-Request or Access-
      Challenge packet, they MUST be in order and they MUST be
      consecutive attributes in the Access-Request or Access-Challenge
      packet.  Access-Accept and Access-Reject packets SHOULD only have
      ONE EAP-Message attribute in them, containing EAP-Success or EAP-
      Failure.

      It is expected that EAP will be used to implement a variety of
      authentication methods, including methods involving strong
      cryptography. In order to prevent attackers from subverting EAP by
      attacking RADIUS/EAP, (for example, by modifying the EAP-Success
      or EAP-Failure packets) it is necessary that RADIUS/EAP provide
      integrity protection at least as strong as those used in the EAP
      methods themselves.

      Therefore the Message-Authenticator attribute MUST be used to
      protect all Access-Request, Access-Challenge, Access-Accept, and
      Access-Reject packets containing an EAP-Message attribute.

      Access-Request packets including an EAP-Message attribute without
      a Message-Authenticator attribute SHOULD be silently discarded by
      the RADIUS server.  A RADIUS Server supporting EAP-Message MUST
      calculate the correct value of the Message-Authenticator and
      silently discard the packet if it does not match the value sent.
      A RADIUS Server not supporting EAP-Message MUST return an Access-
      Reject if it receives an Access-Request containing an EAP-Message
      attribute. A RADIUS Server receiving an EAP-Message attribute that
      it does not understand MUST return an Access-Reject.




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      Access-Challenge, Access-Accept, or Access-Reject packets
      including an EAP-Message attribute without a Message-Authenticator
      attribute SHOULD be silently discarded by the NAS. A NAS
      supporting EAP-Message MUST calculate the correct value of the
      Message-Authenticator and silently discard the packet if it does
      not match the value sent.

   A summary of the EAP-Message attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      79 for EAP-Message.

   Length

      &gt;= 3

   String

      The String field contains EAP packets, as defined in [3].  If
      multiple EAP-Message attributes are present in a packet their
      values should be concatenated; this allows EAP packets longer than
      253 octets to be passed by RADIUS.

<h2><a name='s5.14.'>5.14.  Message-Authenticator</a></h2>

   Description

      This attribute MAY be used to sign Access-Requests to prevent
      spoofing Access-Requests using CHAP, ARAP or EAP authentication
      methods.  It MAY be used in any Access-Request.  It MUST be used
      in any Access-Request, Access-Accept, Access-Reject or Access-
      Challenge that includes an EAP-Message attribute.

      A RADIUS Server receiving an Access-Request with a Message-
      Authenticator Attribute present MUST calculate the correct value
      of the Message-Authenticator and silently discard the packet if it
      does not match the value sent.






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      A RADIUS Client receiving an Access-Accept, Access-Reject or
      Access-Challenge with a Message-Authenticator Attribute present
      MUST calculate the correct value of the Message-Authenticator and
      silently discard the packet if it does not match the value sent.

      Earlier drafts of this memo used &quot;Signature&quot; as the name of this
      attribute, but Message-Authenticator is more precise.  Its
      operation has not changed, just the name.

   A summary of the Message-Authenticator attribute format is shown
   below.  The fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      80 for Message-Authenticator

   Length

      18

   String

      When present in an Access-Request packet, Message-Authenticator is
      an HMAC-MD5 [9] checksum of the entire Access-Request packet,
      including Type, ID, Length and authenticator, using the shared
      secret as the key, as follows.

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Request Authenticator, Attributes)

      When the checksum is calculated the signature string should be
      considered to be sixteen octets of zero.

      For Access-Challenge, Access-Accept, and Access-Reject packets,
      the Message-Authenticator is calculated as follows, using the
      Request-Authenticator from the Access-Request this packet is in
      reply to:

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Request Authenticator, Attributes)





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      When the checksum is calculated the signature string should be
      considered to be sixteen octets of zero.  The shared secret is
      used as the key for the HMAC-MD5 hash.  The is calculated and
      inserted in the packet before the Response Authenticator is
      calculated.

      This attribute is not needed if the User-Password attribute is
      present, but is useful for preventing attacks on other types of
      authentication.  This attribute is intended to thwart attempts by
      an attacker to setup a &quot;rogue&quot; NAS, and perform online dictionary
      attacks against the RADIUS server.  It does not afford protection
      against &quot;offline&quot; attacks where the attacker intercepts packets
      containing (for example) CHAP challenge and response, and performs
      a dictionary attack against those packets offline.

      IP Security will eventually make this attribute unnecessary, so it
      should be considered an interim measure.

<h2><a name='s5.15.'>5.15.  ARAP-Challenge-Response</a></h2>

   Description

      This attribute is sent in an Access-Accept packet with Framed-
      Protocol of ARAP, and contains the response to the dial-in
      client's challenge.

   A summary of the ARAP-Challenge-Response attribute format is shown
   below.  The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Value...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      84 for ARAP-Challenge-Response.

   Length

      10





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   Value

      The Value field contains an 8 octet response to the dial-in
      client's challenge. The RADIUS server calculates this value by
      taking the dial-in client's challenge from the high order 8 octets
      of the ARAP-Password attribute and  performing DES encryption on
      this value with the authenticating user's password as the key. If
      the user's password is less than 8 octets in length, the password
      is padded at the end with NULL octets to a length of 8 before
      using it as a key.

<h2><a name='s5.16.'>5.16.  Acct-Interim-Interval</a></h2>

   Description

      This attribute indicates the number of seconds between each
      interim update in seconds  for this specific session. This value
      can only appear in the Access-Accept message.

   A summary of the Acct-Interim-Interval attribute  format  is  shown
   below. The fields are transmitted from left to right.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      85 for Acct-Interim-Interval.

   Length

      6

   Value

      The Value field contains the number of seconds between each
      interim update to be sent from the NAS for this session. The value
      MUST NOT be smaller than 60.  The value SHOULD NOT be smaller than
      600, and careful consideration should be given to its impact on
      network traffic.






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<h2><a name='s5.17.'>5.17.  NAS-Port-Id</a></h2>

   Description

      This Attribute contains a text string which identifies the port of
      the NAS which is authenticating the user.  It is only used in
      Access-Request and Accounting-Request packets.  Note that this is
      using &quot;port&quot; in its sense of a physical connection on the NAS, not
      in the sense of a TCP or UDP port number.

      Either NAS-Port or NAS-Port-Id SHOULD be present in an Access-
      Request packet, if the NAS differentiates among its ports.  NAS-
      Port-Id is intended for use by NASes which cannot conveniently
      number their ports.

   A summary of the NAS-Port-Id Attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Text...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      87 for NAS-Port-Id.

   Length

      &gt;= 3

   Text

      The Text field contains the name of the port using UTF-8 encoded
      10646 [8] characters.

<h2><a name='s5.18.'>5.18.  Framed-Pool</a></h2>

   Description

      This Attribute contains the name of an assigned address pool that
      SHOULD be used to assign an address for the user.  If a NAS does
      not support multiple address pools, the NAS should ignore this
      Attribute.  Address pools are usually used for IP addresses, but
      can be used for other protocols if the NAS supports pools for
      those protocols.



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   A summary of the Framed-Pool Attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      88 for Framed-Pool

   Length

      &gt;= 3

   String

      The string field contains the name of an assigned address pool
      configured on the NAS.

<h2><a name='s5.19.'>5.19.  Table of Attributes</a></h2>

   The following table provides a guide to which attributes may be found
   in which kind of packets.  Acct-Input-Gigawords, Acct-Output-
   Gigawords, Event-Timestamp, and NAS-Port-Id may have 0-1 instances in
   an Accounting-Request packet.  Connect-Info may have 0+ instances in
   an Accounting-Request packet.  The other attributes added in this
   document must not be present in an Accounting-Request.

Request  Accept  Reject  Challenge   #    Attribute
0-1      0       0       0           70   ARAP-Password [Note 1]
0        0-1     0       0-1         71   ARAP-Features
0        0-1     0       0           72   ARAP-Zone-Access
0-1      0       0       0-1         73   ARAP-Security
0+       0       0       0+          74   ARAP-Security-Data
0        0       0-1     0           75   Password-Retry
0        0       0       0-1         76   Prompt
0-1      0       0       0           77   Connect-Info
0        0+      0       0           78   Configuration-Token
0+       0+      0+      0+          79   EAP-Message [Note 1]
0-1      0-1     0-1     0-1         80   Message-Authenticator [Note 1]
0        0-1     0       0-1         84   ARAP-Challenge-Response
0        0-1     0       0           85   Acct-Interim-Interval
0-1      0       0       0           87   NAS-Port-Id
0        0-1     0       0           88   Framed-Pool
Request  Accept  Reject  Challenge   #    Attribute



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   [Note 1] An Access-Request that contains either a User-Password or
   CHAP-Password or ARAP-Password or one or more EAP-Message attributes
   MUST NOT contain more than one type of those four attributes.  If it
   does not contain any of those four attributes, it SHOULD contain a
   Message-Authenticator.  If any packet type contains an EAP-Message
   attribute it MUST also contain a Message-Authenticator.

   The following table defines the above table entries.

      0     This attribute MUST NOT be present
      0+    Zero or more instances of this attribute MAY be present.
      0-1   Zero or one instance of this attribute MAY be present.
      1     Exactly one instance of this attribute MUST be present.

<h2><a name='s6.'>6.  IANA Considerations</a></h2>

   The Packet Type Codes, Attribute Types, and Attribute Values defined
   in this document are registered by the Internet Assigned Numbers
   Authority (IANA) from the RADIUS name spaces as described in the
   &quot;IANA Considerations&quot; section of [1], in accordance with <A HREF="http://RFC.net/bcp26.html">BCP 26</A> [10].

<h2><a name='s7.'>7.  Security Considerations</a></h2>

   The attributes other than Message-Authenticator and EAP-Message in
   this document have no additional security considerations beyond those
   already identified in [1].

<h2><a name='s7.1.'>7.1.  Message-Authenticator Security</a></h2>

   Access-Request packets with a User-Password establish the identity of
   both the user and the NAS sending the Access-Request, because of the
   way the shared secret between NAS and RADIUS server is used.
   Access-Request packets with CHAP-Password or EAP-Message do not have
   a User-Password attribute, so the Message-Authenticator attribute
   should be used in access-request packets that do not have a User-
   Password, in order to establish the identity of the NAS sending the
   request.

<h2><a name='s7.2.'>7.2.  EAP Security</a></h2>

   Since the purpose of EAP is to provide enhanced security for PPP
   authentication, it is critical that RADIUS support for EAP be secure.
   In particular, the following issues must be addressed:

      Separation of EAP server and PPP authenticator
      Connection hijacking
      Man in the middle attacks
      Multiple databases



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      Negotiation attacks

<h2><a name='s7.2.1.'>7.2.1.  Separation of EAP server and PPP authenticator</a></h2>

   It is possible for the EAP endpoints to mutually authenticate,
   negotiate a ciphersuite, and derive a session key for subsequent use
   in PPP encryption.

   This does not present an issue on the peer, since the peer and EAP
   client reside on the same machine; all that is required is for the
   EAP client module to pass the session key to the PPP encryption
   module.

   The situation is more complex when EAP is used with RADIUS, since the
   PPP authenticator will typically not reside on the same machine as
   the EAP server. For example, the EAP server may be a backend security
   server, or a module residing on the RADIUS server.

   In the case where the EAP server and PPP authenticator reside on
   different machines, there are several implications for security.
   Firstly, mutual authentication will occur between the peer and the
   EAP server, not between the peer and the authenticator. This means
   that it is not possible for the peer to validate the identity of the
   NAS or tunnel server that it is speaking to.

   As described earlier, when EAP/RADIUS is used to encapsulate EAP
   packets, the Message-Authenticator attribute is required in
   EAP/RADIUS Access-Requests sent from the NAS or tunnel server to the
   RADIUS server. Since the Message-Authenticator attribute involves a
   HMAC-MD5 hash, it is possible for the RADIUS server to verify the
   integrity of the Access-Request as well as the NAS or tunnel server's
   identity.  Similarly, Access-Challenge packets sent from the RADIUS
   server to the NAS are also authenticated and integrity protected
   using an HMAC-MD5 hash, enabling the NAS or tunnel server to
   determine the integrity of the packet and verify the identity of the
   RADIUS server.  Moreover, EAP packets sent via methods that contain
   their own integrity protection cannot be successfully modified by a
   rogue NAS or tunnel server.

   The second issue that arises in the case of an EAP server and PPP
   authenticator residing on different machines is that the session key
   negotiated between the peer and EAP server will need to be
   transmitted to the authenticator.  Therefore a mechanism needs to be
   provided to transmit the session key from the EAP server to the
   authenticator or tunnel server that needs to use the key. The
   specification of this transit mechanism is outside the scope of this
   document.




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<h2><a name='s7.2.2.'>7.2.2.  Connection hijacking</a></h2>

   In this form of attack, the attacker attempts to inject packets into
   the conversation between the NAS and the RADIUS server, or between
   the RADIUS server and the backend security server. RADIUS does not
   support encryption, and as described in [1], only Access-Reply and
   Access-Challenge packets are integrity protected. Moreover, the
   integrity protection mechanism described in [1] is weaker than that
   likely to be used by some EAP methods, making it possible to subvert
   those methods by attacking EAP/RADIUS.

   In order to provide for authentication of all packets in the EAP
   exchange, all EAP/RADIUS packets MUST be authenticated using the
   Message-Authenticator attribute, as described previously.

<h2><a name='s7.2.3.'>7.2.3.  Man in the middle attacks</a></h2>

   Since RADIUS security is based on shared secrets, end-to-end security
   is not provided in the case where authentication or accounting
   packets are forwarded along a proxy chain.  As a result, attackers
   gaining control of a RADIUS proxy will be able to modify EAP packets
   in transit.

<h2><a name='s7.2.4.'>7.2.4.  Multiple databases</a></h2>

   In many cases a backend security server will be deployed along with a
   RADIUS server in order to provide EAP services. Unless the backend
   security server also functions as a RADIUS server, two separate user
   databases will exist, each containing information about the security
   requirements for the user. This represents a weakness, since security
   may be compromised by a successful attack on either of the servers,
   or their backend databases. With multiple user databases, adding a
   new user may require multiple operations, increasing the chances for
   error.  The problems are further magnified in the case where user
   information is also being kept in an LDAP server. In this case, three
   stores of user information may exist.

   In order to address these threats, consolidation of databases is
   recommended.  This can be achieved by having both the RADIUS server
   and backend security server store information in the same backend
   database; by having the backend security server provide a full RADIUS
   implementation; or by consolidating both the backend security server
   and the RADIUS server onto the same machine.








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<h2><a name='s7.2.5.'>7.2.5.  Negotiation attacks</a></h2>

   In a negotiation attack, a rogue NAS, tunnel server, RADIUS proxy or
   RADIUS server causes the authenticating peer to choose a less secure
   authentication method so as to make it easier to obtain the user's
   password. For example, a session that would normally be authenticated
   with EAP would instead authenticated via CHAP or PAP; alternatively,
   a connection that would normally be authenticated via one EAP type
   occurs via a less secure EAP type, such as MD5. The threat posed by
   rogue devices, once thought to be remote, has gained currency given
   compromises of telephone company switching systems, such as those
   described in [11].

   Protection against negotiation attacks requires the elimination of
   downward negotiations. This can be achieved via implementation of
   per-connection policy on the part of the authenticating peer, and
   per-user policy on the part of the RADIUS server.

   For the authenticating peer, authentication policy should be set on a
   per-connection basis. Per-connection policy allows an authenticating
   peer to negotiate EAP when calling one service, while negotiating
   CHAP for another service, even if both services are accessible via
   the same phone number.

   With per-connection policy, an authenticating peer will only attempt
   to negotiate EAP for a session in which EAP support is expected. As a
   result, there is a presumption that an authenticating peer selecting
   EAP requires that level of security. If it cannot be provided, it is
   likely that there is some kind of misconfiguration, or even that the
   authenticating peer is contacting the wrong server. Should the NAS
   not be able to negotiate EAP, or should the EAP-Request sent by the
   NAS be of a different EAP type than what is expected, the
   authenticating peer MUST disconnect. An authenticating peer expecting
   EAP to be negotiated for a session MUST NOT negotiate CHAP or PAP.

   For a NAS, it may not be possible to determine whether a user is
   required to authenticate with EAP until the user's identity is known.
   For example, for shared-uses NASes it is possible for one reseller to
   implement EAP while another does not. In such cases, if any users of
   the NAS MUST do EAP, then the NAS MUST attempt to negotiate EAP for
   every call. This avoids forcing an EAP-capable client to do more than
   one authentication, which weakens security.

   If CHAP is negotiated, the NAS will pass the User-Name and CHAP-
   Password attributes to the RADIUS Server in an Access-Request packet.
   If the user is not required to use EAP, then the RADIUS Server will
   respond with an Access-Accept or Access-Reject packet as appropriate.
   However, if CHAP has been negotiated but EAP is required, the RADIUS



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   server MUST respond with an Access-Reject, rather than an Access-
   Challenge/EAP-Message/EAP-Request packet.  The authenticating peer
   MUST refuse to renegotiate authentication, even if the renegotiation
   is from CHAP to EAP.

   If EAP is negotiated but is not supported by the RADIUS proxy or
   server, then the server or proxy MUST respond with an Access-Reject.
   In these cases, the NAS MUST send an LCP-Terminate and disconnect the
   user.  This is the correct behavior since the authenticating peer is
   expecting EAP to be negotiated, and that expectation cannot be
   fulfilled. An EAP-capable authenticating peer MUST refuse to
   renegotiate the authentication protocol if EAP had initially been
   negotiated.  Note that problems with a non-EAP capable RADIUS proxy
   could prove difficult to diagnose, since a user dialing in from one
   location (with an EAP-capable proxy) might be able to successfully
   authenticate via EAP, while the same user dialing into another
   location (and encountering an EAP-incapable proxy) might be
   consistently disconnected.

<h2><a name='s8.'>8.  References</a></h2>

   [1]  Rigney, C., Willens, S., Rubens, A. and W. Simpson, &quot;Remote
        Authentication Dial In User Service (RADIUS)&quot;, <A HREF="http://RFC.net/rfc2865.html">RFC 2865</A>, June
        2000.

   [2]  Rigney, C., &quot;RADIUS Accounting&quot;, <A HREF="http://RFC.net/rfc2866.html">RFC 2866</A>, June 2000.

   [3]  Blunk, L. and J. Vollbrecht, &quot;PPP Extensible Authentication
        Protocol (EAP)&quot;, <A HREF="http://RFC.net/rfc2284.html">RFC 2284</A>, March 1998.

   [4]  Bradner, S., &quot;Key words for use in RFCs to Indicate Requirement
        Levels&quot;, <A HREF="http://RFC.net/bcp14.html">BCP 14</A>, <A HREF="http://RFC.net/rfc2119.html">RFC 2119</A>, March, 1997.

   [5]  Reynolds, J. and J. Postel, &quot;Assigned Numbers&quot;, <A HREF="http://RFC.net/std2.html">STD 2</A>, <A HREF="http://RFC.net/rfc1700.html">RFC 1700</A>,
        October 1994.

   [6]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.  and
        <a href='#sI.'>I.</a> Goyret, &quot;RADIUS Attributes for Tunnel Protocol Support&quot;, RFC
        2868, June 2000.

   [7]  Zorn, G., Aboba, B. and D. Mitton, &quot;RADIUS Accounting
        Modifications for Tunnel Protocol Support&quot;, <A HREF="http://RFC.net/rfc2867.html">RFC 2867</A>, June 2000.

   [8]  Yergeau, F., &quot;UTF-8, a transformation format of ISO 10646&quot;, RFC
        2279, January 1998.






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   [9]  Krawczyk, H., Bellare, M. and R. Canetti, &quot;HMAC: Keyed-Hashing
        for Message Authentication&quot;, <A HREF="http://RFC.net/rfc2104.html">RFC 2104</A>, February 1997.

   [10] Alvestrand, H. and T. Narten, &quot;Guidelines for Writing an IANA
        Considerations Section in RFCs&quot;, <A HREF="http://RFC.net/bcp26.html">BCP 26</A>, <A HREF="http://RFC.net/rfc2434.html">RFC 2434</A>, October 1998.

   [11] Slatalla, M., and  Quittner, J., &quot;Masters of Deception.&quot;
        HarperCollins, New York, 1995.

<h2><a name='s9.'>9.  Acknowledgements</a></h2>

   RADIUS and RADIUS Accounting were originally developed by Livingston
   Enterprises (now part of Lucent Technologies) for their PortMaster
   series of Network Access Servers.

   The section on ARAP is adopted with permission from &quot;Using RADIUS to
   Authenticate Apple Remote Access Connections&quot; by Ward Willats of Cyno
   Technologies (ward@cyno.com).

   The section on Acct-Interim-Interval is adopted with permission from
   an earlier work in progress by Pat Calhoun of Sun Microsystems, Mark
   Beadles of Compuserve, and Alex Ratcliffe of UUNET Technologies.

   The section on EAP is adopted with permission from an earlier work in
   progress by Pat Calhoun of Sun Microsystems, Allan Rubens of Merit
   Network, and Bernard Aboba of Microsoft.  Thanks also to Dave Dawson
   and Karl Fox of Ascend, and Glen Zorn and Narendra Gidwani of
   Microsoft for useful discussions of this problem space.

<h2><a name='s10.'>10.  Chair's Address</a></h2>

   The RADIUS working group can be contacted via the current chair:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   Phone: +1 925 737 2100
   EMail: cdr@telemancy.com











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<h2><a name='s11.'>11.  Authors' Addresses</a></h2>

   Questions about this memo can also be directed to:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   EMail: cdr@telemancy.com

   Questions on ARAP and RADIUS may be directed to:

   Ward Willats
   Cyno Technologies
   1082 Glen Echo Ave
   San Jose, CA 95125

   Phone: +1 408 297 7766
   EMail: ward@cyno.com































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   Questions on EAP and RADIUS may be directed to any of the following:

   Pat R. Calhoun
   Network and Security Research Center
   Sun Microsystems, Inc.
   15 Network Circle
   Menlo Park, CA 94025

   Phone: +1 650 786 7733
   EMail: pcalhoun@eng.sun.com


   Allan C. Rubens
   Tut Systems, Inc.
   220 E. Huron, Suite 260
   Ann Arbor, MI 48104

   Phone: +1 734 995 1697
   EMail: arubens@tutsys.com


   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 936 6605
   EMail: bernarda@microsoft.com























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<h2><a name='s12.'>12.  Full Copyright Statement</a></h2>

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   &quot;AS IS&quot; basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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